In an electric double layer capacitor using an organic solvent type electrolyte, lowering in withstand voltage or voltage maintaining rate, etc. occurs even when a minute amount of water is migrated therein to cause bad influences on capacitor characteristics, so that it has generally been carried out to remove the water component contained these members by drying them at a high temperature for a long period of time in a state that electrodes and a separator are wound or laminated together. Therefore, it has been required for the separator to have heat resistance which can endure a high temperature atmosphere for a long period of time. In the preparation steps of an aluminum solid electrolytic capacitor using a conductive polymer as a dielectric material, there is a so-called re-conversion treatment in which an aluminum foil subjected to conversion treatment and a separator are wound or laminated together and it is subjected to a high temperature heat treatment in such a state, so that the separator is required to have heat-resistance. Moreover, after the re-conversion treatment, there is a step of impregnating a monomer solution of a conductive polymer and polymerizing the same, so that the separator is required to have a monomer solution permeability of the conductive polymer after the high temperature heat treatment. There may be mentioned, for example, a separator for an electric double layer capacitor comprising a fiber sheet which contains fiber having fibril and fine polyester fiber with a fineness of 0.45 dtex or less (for example, see Patent Literature 1), an electrolytic capacitor comprising a separator which comprises polyamide fiber as a main fiber (for example, see Patent Literature 2), etc.
However, there is a problem that these separators are dried at a temperature higher than a softening temperature of the fiber constituting the separator for a long period of time, they do not work as a separator due to softening or heat deterioration of the fiber, or a problem that formation of a conductive polymer film becomes difficult since permeability of a monomer solution for the conductive polymer is lost.    [Patent Literature 1] Japanese Unexamined Patent Publication No. 2001-244150    [Patent Literature 2] Japanese Unexamined Patent Publication No. 2002-198263